Tertiary alkoxyl radicals from 3-alkoxythiazole-2(3H)-thiones

This study deals with the synthesis of tert-O-alkyl thiohydroxamates and their use as tert-alkoxyl radical precursors. tert-Alkoxyl radicals were applied in mechanistic studies to determine rate constants of (i) p-chlorocumyloxyl radical addition to bicyclo[2.2.1]heptene (k=1×10⁷ M⁻¹ s⁻¹), (ii) 2-phenylhex-5-en-2-oxyl radical 5-exo-trig-cyclization (k^cis=3×10⁹ s⁻¹, k^trans=1×10⁹ s⁻¹), and (iii) 2-methyl-5-phenylpent-2-oxyl to 2-hydroxy-2-methyl-5-phenylpent-5-yl radical isomerization (1,5-H-atom shift; k=0.4-1.5×10⁸ s⁻¹). The reactions pose key steps in synthesis of 2,2,5-substituted tetrahydrofurans and 2-bromo-3-alkoxybicyclo[2.2.1]heptanes. Stereoselectivity in 5-exo-trig cyclization (2,5-cis) and intermolecular addition (exo/endo>99:1), originates from torsional strain in transition structures of alkoxyl radical reactions.